<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>验证输入</title>
</head>

<body>
    <h1>我的第一段 JavaScript</h1>
    <p>请输入数字。如果输入值不是数字，浏览器会弹出提示框。</p>
    <input id="demo" type="text">
    <script>
        function myFunction() {
            var x = document.getElementById("demo").value;
            if (x == "" || isNaN(x)) {
                alert("不是数字");
            }
        }
    </script>
    <button type="button" onclick="myFunction()">点击这里</button>
</body>

</html>